The soil microbe Bacillus thuringiensis (B.t.) is a Gram-positive, spore-forming bacterium characterized by parasporal crystalline protein inclusions. These inclusions often appear microscopically as distinctively shaped crystals. The proteins can be highly toxic to pests and specific in their toxic activity. Certain B.t. toxin genes have been isolated and sequenced, and recombinant DNA-based B.t. products have been produced and approved for use. In addition, with the use of genetic engineering techniques, new approaches for delivering B.t. endotoxins to agricultural environments are under development, including the use of plants genetically engineered with endotoxin genes for insect resistance and the use of stabilized intact microbial cells as B.t. endotoxin delivery vehicles (Gaertner, F. H., L. Kim [1988] TIBTECH 6:S4-7). Thus, isolated B.t. endotoxin genes are becoming commercially valuable.
Until the last ten years, commercial use of B.t. pesticides has been largely restricted to a narrow range of lepidopteran (caterpillar) pests. Preparations of the spores and crystals of B. thuringiensis subsp. kurstaki have been used for many years as commercial insecticides for lepidopteran pests. For example, B. thuringiensis var. kurstaki HD-1 produces a crystal called a .delta.-endotoxin which is toxic to the larvae of a number of lepidopteran insects.
In recent years, however, investigators have discovered B.t. pesticides with specificities for a much broader range of pests. For example, other species of B.t., namely israelensis and tenebrionis (a.k.a. B.t. M-7, a.k.a. B.t. san diego), have been used commercially to control insects of the orders Diptera and Coleoptera, respectively (Gaertner, F. H. [1989] "Cellular Delivery Systems for Insecticidal Proteins: Living and Non-Living Microorganisms," in Controlled Delivery of Crop Protection Agents, R. M. Wilkins, ed., Taylor and Francis, New York and London, 1990, pp. 245-255). See also Couch, T. L. (1980) "Mosquito Pathogenicity of Bacillus thuringiensis var. israelensis," Developments in Industrial Microbiology 22:61-76; Beegle, C. C., (1978) "Use of Entomogenous Bacteria in Agroecosystems," Developments in Industrial Microbiology 20:97-104. Krieg, A., A. M. Huger, G. A. Langenbruch, W. Schnetter (1983) Z. ang. Ent. 96:500-508, describe Bacillus thuringiensis var. tenebrionis, which is reportedly active against two beetles in the order Coleoptera. These are the Colorado potato beetle, Leptinotarsa decemlineata, and Agelastica alni.
Recently, new subspecies of B.t. have been identified, and genes responsible for active .delta.-endotoxin proteins have been isolated (Hofte, H., H. R. Whiteley [1989] Microbiological Reviews 52(2):242-255). Hofte and Whiteley classified B.t. crystal protein genes into 4 major classes. The classes were CryI (Lepidoptera-specific), CrylI (Lepidoptera- and Diptera-specific), CrylII (Coleoptera-specific), and CryIV (Diptera-specific). The discovery of strains specifically toxic to other pests has been reported. (Feitelson, J. S., J. Payne, L. Kim [1992] Bio/Technology 10:271-275).
The cloning and expression of a B.t. crystal protein gene in Escherichia coli has been described in the published literature (Schnepf, H. E., H. R. Whitely [1981] Proc. Natl. Acad. Sci. USA 78:2893-2897). U.S. Pat. Nos. 4,448,885 and 4,467,036 both disclose the expression of B.t. crystal protein in E. coli. U.S. Pat. Nos. 4,797,276 and 4,853,331 disclose B. thuringiensis strain san diego (a.k.a. B.t. tenebrionis, a.k.a. M-7) which can be used to control coleopteran pests in various environments. U.S. Pat. No. 4,918,006 discloses B.t. toxins having activity against Dipterans. U.S. Pat. No. 4,849,217 discloses B.t. isolates which have activity against the alfalfa weevil. U.S. Pat. Nos. 5,151,363 and 4,948,734 disclose certain isolates of B.t. which have activity against nematodes. As a result of extensive research and investment of resources, other patents have issued for new B.t. isolates and new uses of B.t. isolates. However, the discovery of new B.t. isolates and new uses of known B.t. isolates remains an empirical, unpredictable art.
Dipteran insects are serious nuisances as well as being vectors of many human and animal diseases such as malaria, onchocerciasis, equine encephalitis, and dog heartworm. The activity spectrum of B.t. .delta.-endotoxins to insects of the order Diptera includes activity against mosquitoes as well as black flies. See Couch, supra; Beegle, supra.
The two varieties of B.t. known to kill mosquitos and blackflies are B.t. israelensis (B.t.i.) (Goldberg, L. J., J. Margalit [1977] Mosquito News 37:355-358) and B.t. morrisoni (B.t.m.) (Padua, L. E., M. Ohba, K. Aizawa [1984] J. Invertebrate Pathology 44:12-17). These B.t. are not harmful to non-target organisms (Mulla, M. S., B. A. Federici, H. A. Darwazeh [1982] Environmental Entomology 11:788-795), and play an important role in the integrated management of dipteran pests. They are safe to use in urban areas, and can be used in aquatic environments without harm to other species.
Dipteran pests are also a major problem in the poultry and cattle industries. The horn fly, a serious cattle pest, is killed by B.t. in the larval stages (Temeyer, K. B. [1990] "Potential of Bacillus thuringiensis for fly control," Fifth International Colloquium on Invertebrate Pathology and Microbial Control, Society for Invertebrate Pathology, 352-356). European Patent Application 90307204.9 (Publication No. 0 409 438) discloses Bacillus thuringiensis dipteran-active isolates PS71M3 and PS123D1.
Flies are an abundant species that can be found almost everywhere. They usually occur in such large numbers as to constitute a nuisance. The majority of the Diptera are considered pests and are of economic importance. A number of adult species are blood-sucking and cause irritation to man and domestic animals. Others are scavenging flies that mechanically transmit organisms and pathogens that contaminate food. Both types of flies are important vectors of disease, such as malaria, yellow fever, filariasis, sleeping sickness, typhoid fever, and dysentery. Larvae of a few species are pests of major agriculture crops. The larvae can feed on all parts of the plant such as seeds, roots, leaves and fruits. Larvae of certain species feed on fungus causing damage to mushroom production. Larvae can irritate domestic animals when they develop in the animal. Both the adults and larval forms of dipterans are considered pests to man and in agriculture.
House flies (family Museidae) are an important pest from the order Diptera. They are considered a nuisance and are vectors of human and animal diseases. Their habits of walking and feeding on garbage and excrement and on the human person and food make them ideal agents for the transfer of disease (Metcalf, C. and Flint, W. 1962. Destructive and Useful Insects, McGraw-Hill Book Co., N.Y., pp. 1030-1035). House flies are also a pest to animals and transmit disease through open wounds. The family Muscidae also includes the little house fly, face fly, stable fly, and horn fly, all of which are pests of livestock. These species are pests of cattle, poultry, horses and other types of livestock. They breed in manure and decaying straw located near the animals. The horn and stable flies are biting flies which cause stress to dairy cattle reducing milk production. The family Muscidae is considered an economic problem domestically and worldwide.
Leafmining flies cause damage and yield loss to economically important crops such as potatoes, tomatoes and celery. Dipteran leafminers are also considered a major pest in the ornamental flower industry (Parrella, M. P. [1987] "Biology of Liriomyza," Ann. Rev. Entomol. 32:201-224). The most common leafminers are found in the family Agromyzidae although the families Anthomyiidae, Drosophilidae and Ephydridae also contain leafmining flies (Hespenheide, H. A. [1991] "Bionomics of leafmining insects," Ann. Rev. Entomolo. 36:535-60). Flies in the genus Liriomyza (also known as serpentine leafminers) are particularly important because of their worldwide distribution, polyphagous nature and resistance to insecticides. In the state of California, the chrysanthemum industry lost approximately 93 million dollars to Liriomyza trifolii between the years of 1981-1985.
There are also dipteran pests of plants, such as Hessian fly, Medfly, and Mexfly, for which a B.t. product would be very valuable.
Another serious pest to plants is the corn rootworm. The corn rootworm is a coleopteran pest. Extensive damage occurs to the United States corn crop each year due to root feeding by larvae of corn rootworm (Diabrotica spp.). Three main species of corn rootworm, Western corn rootworm (Diabrotica virgifera virgifera), Northern corn rootworm (Diabrotica barberi), and Southern corn rootworm (Diabrotica undecimpunctata howardi) cause varying degrees of damage to corn in the United States. It has been estimated that the annual cost of insecticides to control corn rootworm and the annual crop losses caused by corn rootworm damage exceeds a total of $1 billion in the United States each year (Meycalf, R. L. [1986] in Methods for the Study of Pest Diabrotica, Drysan, J. L. and T. A. Miller [Eds.], Springer-Verlag, New York, N.Y., pp. vii-xv). Approximately $250 million worth of insecticides are applied annually to control corn rootworms in the United States. In the Midwest, $60 million and $40 million worth of insecticide were applied in Iowa and Nebraska, respectively, in 1990. Even with insecticide use, rootworms cause about $750 million worth of crop damage each year, making them the most serious corn insect pest in the Midwest.
The life cycle of each Diabrotica species is similar. The eggs of the corn rootworm are deposited in the soft. Newly hatched larvae (the first instar) remain in the ground and feed on the smaller branching corn roots. Later instars of Western and Northern corn rootworms invade the inner root tissues that transport water and mineral elements to the plants. In most instances, larvae migrate to feed on the newest root growth. Tunneling into roots by the larve results in damage which can be observed as brown, elongated scars on the root surface, tunneling within the roots, or varying degrees of pruning. Plants with pruned roots usually dislodge after storms that are accompanied by heavy rains and high winds. The larvae of Southern corn rootworm feed on the roots in a similar manner as the Western and Northern corn rootworm larvae. Southern corn rootworm larvae may also feed on the growing point of the stalk while it is still near the soil line, which may cause the plant to wilt and die.
After feeding for about 3 weeks, the corn rootworm larvae leave the roots and pupate in the soil. The adult beetles emerge from the soil and may feed on corn pollen and many other types of pollen, as well as on corn silks. Feeding on green silks can reduce pollination level, resulting in poor grain set and poor yield. The Western corn rootworm adult also feeds upon corn leaves, which can slow plant growth and, on rare occasions, kill plants of some corn varieties.
Current methods for controlling corn rootworm damage in corn are limited to the use of crop rotation and insecticide application. However, economic demands on the utilization of farmland restrict the use of crop rotation. In addition, an emerging two-year diapause (or overwintering) trait of Northern corn rootworms is disrupting crop rotations in some areas.
The use of insecticides to control corn rootworm also has several drawbacks. Continual use of insecticides has allowed resistant insects to evolve. Situations such as extremely high populations of larve, heavy rains, and improper calibration of insecticide application equipment can result in poor control. Insecticide use often raises environmental concerns such as contamination of soil and of both surface and underground water supplies. Working with insecticides may also pose hazards to the persons applying them.